Semiconductor memories can be classified into volatile memory and non-volatile memory depending on the retention of data when the power is turned off. A direct random access memory (DRAM) and a static random access memory (SRAM) are exemplary representative volatile memories, and a flash memory is one exemplary representative non-volatile memory. These typical RAMs have a logic value of ‘0’ or ‘1’ depending on the presence or absence of charges stored thereon so as to provide various functions as a memory.
In general, DRAMs, which are volatile memories, require periodic refresh and high charge storage capability (i.e., capacitance). Many attempts have been made to increase the capacitance of DRAMs. For instance, the surface area of a capacitor electrode may be increased to increase its capacitance. However, increasing the surface area of the capacitor electrode may cause a limitation in increasing the scale of integration of DRAMs.
On the other hand, a flash memory is usually configured with gate structures, each including a gate insulation layer, a floating gate, a dielectric layer, and a control gate, which are formed on a substrate in sequential order. A flash memory writes or erases data by charge tunneling through the gate insulation layer. The writing and erasing operations usually require an operation voltage higher than a power supply voltage. Thus, a flash memory generally requires a boosting circuit to generate a voltage necessary for writing and erasing data.
Intensive research has been and is being conducted to develop a memory that, while having the characteristics of non-volatile memories, allows certain access and increases the scale of integration yet retains a simple structure. A phase change RAM (PRAM) is one representative example of such a memory. In general, a phase change memory uses a phase change material whose phase changes depending on heat supplied thereto. A chalcogen compound including germanium (Ge), antimony (Sb) and tellurium (Te), which is typically labeled as GST or Ge—Sb—Te, may be used as the phase change material. Current flows through a layer of the phase change material to generate heat in the phase change material. Depending on the amount of current and the supply time, a phase change may result in the phase change material (e.g., GST).
Certain information can be based on the unique properties of the phase change materials used. For example, GST exhibits different levels of resistance depending on its phase, more specifically, GST is characterized by a relatively low resistance when in a crystalline phase and a relatively high resistance when in an amorphous phase. Logic information can be determined based on the resistance difference.
Stated differently, when a current pulse with large magnitude (e.g., resistance heat) is applied to a phase change material layer for a short period of time such that the phase change material layer is heated up to its melting point and then allowed to cool within a short period (e.g., 1ns or less), a portion of the phase change material layer that has been heated changes its phase into an amorphous phase (e.g., a reset state). In contrast, when a current pulse with small magnitude is applied to a phase change material layer for a long period such that it maintains a crystallization temperature lower than the melting point thereof and is then coolled, a portion of the phase change material layer that has been heated changes its phase into a crystalline state (e.g., a set state).
FIG. 1 illustrates a sectional view of a typical phase change memory, e.g., a phase change RAM. A first electrode layer 4 is formed on a substrate 2, and a patterned phase change material layer 6 is formed on the first electrode layer 4. A second electrode 8 is formed on the patterned phase change material layer 6.
However, when the patterned phase change material layer 6 changes its phase from a crystalline phase to an amorphous phase, heat generated in the phase change material layer often causes erroneous operations of the phase change RAM. This may occur because this type of phase change generally requires high energy.
In addition, the volume of a phase change material may not return to the original volume resulting in volume changes with each phase change. Consequently, the lifetime of the phase change memories may be shortened.